The present invention relates to a linearizer for predistorting a signal in order to compensate for non-linearities; and, in particular, to a predistortion linearizer capable of providing a predistorted signal through the use of even order intermodulation components.
Communications systems handling a single carrier usually employ high efficiency and low linearity amplifiers. Multi-carrier systems, however, require highly linear amplifiers at the expense of efficiency. An ideal amplifier would exhibit both perfect linearity and high efficiency.
Intermodulation distortion products are produced by non-linearities in amplifiers handling radio frequency(RF) signals such as the multiple carrier signals found in cellular telephone systems or in various other types of personal communications systems(PCS). Distortions can be caused by amplitude compressions or phase shifts that may occur in relatively large amplitude signals. The resulting spurious signals, when added to the spectrum of the information bearing signals, may degrade the quality of associated communications.
While the phenomena are fairly complex, the transfer function of an amplifier exhibiting non-linearities can be approximated by a Taylor series, that is, a polynominal with terms of the form anxn. The first order effect (alx) is the desired linear function. Distortion is caused by the second order term (square term), the third order term(cubic term), and so forth. For many high frequency systems, the even order terms produce harmonics that are out of the working bandwidth of a system and therefore can be readily discarded. Typically, only odd order (third, fifth, seventh, etc.) intermodulation products reside within the working spectrum of the system and thus are of a concern but with decreasing deleterious effects in that order.
Intermodulation distortion is highly undesirable because it causes interference, crosstalk, and other deleterious effects on the performance of a system employing a high power amplifier. One of the conventional approaches employed to counteract the distortion introduced by an amplifier involves predistortion of an input signal to the amplifier, wherein the input signal to the amplifier is predistorted in such a way that the intentionally added distortion effectively cancels out the distortion generated by the amplifier to thereby provide an undistorted output signal.
Referring to FIG. 1, there is illustrated a conventional predistortion circuit 50 for predistorting a RF signal, which includes two power dividers 10 and 12, an error amplifier 14, vector modulators 16 and 20, a signal canceler 18, a delay line 22 and a combiner 24.
The power dividers 10 and 12 sequentially divide an input RE signal having, e.g., two carriers into three RF signals RF1, RF2 and RF3. The error amplifier 14 amplifies the RF1 to generate a distorted RF signal having therein amplified carriers and intermodulation distortion components. The vector modulator 16 adjusts the amplitude and polarity (i.e., phase) of the RF2 in such a way that the adjusted RF signal has carriers whose amplitudes are substantially identical to those of the amplified carriers of the distorted RF signal, but with a 180 degree phase shift. At the signal canceler 18, the adjusted and the distorted RF signals are combined to extract the intermodulation distortion components. The vector modulator 20 varies the amplitudes and phase of the predistortion components extracted by the canceler 18. The delay line 22 serves to delay the RF signal RF3 to compensate for the time delay required in producing the predistortion components. The combiner 24 combines the delayed RF3 signal with the predistortion components to provide a main amplifier (not shown) with a predistorted RF signal having therein intentionally added distortion components to offset the distortion to be generated by the amplifier itself.
In the predistortion circuit described above, however, the predistortion components are not produced in an independently controlled manner. Therefore, The predistortion should be focused on the cancellation of the most deleterious intermodulation product, i.e., the third order component. Accordingly, if non-linear characteristics of an error amplifier for producing a predistorted signal and a main amplifier for amplifying the signal are different, which would be the case in general, higher order, e.g., fifth and seventh order, intermodulation products may not be effectively removed, deteriorating the overall performance of the system.
It is, therefore, an object of the present invention to provide a linearization circuit capable of providing independently controlled predistorting components.
In accordance with the present invention, there is provided a predistortion linearizer for predistorting a radio frequency(RF) signal having one or, more fundamental frequencies, comprising:
a divider for dividing the RF signal into a first and a second RF signals;
a generator, responsive to the second RF signal, for generating one or more even order harmonics of the fundamental frequencies; and
a producer for producing one or more odd order harmonics from the fundamental frequencies and the even order harmonics to provide a predistorted RF signal including the fundamental frequencies and the odd order harmonics.